Pressurized-water reactors (PWR) in nuclear power plants employ fuel assemblies made of zirconium alloys, of which Zircaloy-4 having the following composition is typical:
1.2-1.7% Sn; PA1 0.18-0.24% Fe; PA1 0.07-0.13% Cr; and PA1 0.2 to 0.9% Sn; PA1 0.18 to 0.6% Fe; PA1 0.07 to 0.4% Cr; PA1 one or both of 0.05 to less than 0.5% Nb and 0.01 to 0.2% Ta; PA1 one or both of 0.05 to 1% V and 0.05 to 1% Mo; and PA1 the balance being zirconium and incidental impurities.
the balance being zirconium and incidental impurities (the percentage being based on weight as hereinafter).
With a view to improving the economy of the operation of nuclear power plants, efforts are being made to maximize the efficiency of fuel burnup and this has led to the need for the fuel assembly to stay in the reactor for a longer period. But this need cannot be met by the conventional fuel assembly made of zirconium alloys, typically Zircaloy-4, because they do not have sufficient corrosion resistance, strength or stress relaxation property (creep characteristics) to withstand prolonged exposure to the atmosphere in the reactor.
Yutaka Matsuo, one of the inventors of the present invention, filed two joint U. S. patent applications (the other inventors were Yoshitaka Suda and Nobuo Suda) on highly corrosion-resistant zirconium alloys for use as a nuclear reactor fuel cladding material claiming priority from Japanese patent applications. One of these applications is U.S. Ser. No. 072,556 (corresponding to Japanese Patent Application No. 61-178441) which was directed at a zirconium alloy consisting essentially of 0.2=1.15% Sn, 0.18-0.24% Fe, 0.07-0.13% Cr, 0.05-1.0% Nb, and the balance Zr. The other application is U.S. Ser. No. 072,411 (corresponding to Japanese Patent Application No. 61-178442) which was directed at a zirconium alloy consisting essentially of 0.2-1.7% Sn, 0.18-0.24% Fe, 0.07-0.13% Cr, 0.01-0.1% Ta, 0.05-1% Nb (optional), and the balance Zr.
The inventors of the present invention also filed Japanese Patent Application No. 63-12323 which was directed at a zirconium alloy for use as a nuclear reactor fuel cladding material that consisted essentially of 0.2-1.15% Sn, 0.18-0.6% Fe, 0.07-0.4% Cr, 0.05-1.0% Nb (optional) and/or 0.01-0.2% Ta (also optional), and the balance Zr.
U.S. Pat. No. 3,121,034 to Anderko et al. describes an alloy that contains up to 3% Sn, 0.5-5% Nb, up to 2% of one of Fe, Ni, Cr, Ta, Pa, Mo and W, and the balance Zr. U.S. Pat. No. 4,649,023 to Sabol et al. describes a zirconium alloy for use as a nuclear reactor fuel cladding material that comprises 0.5-2.0% Nb, 0.9-1.5% Sn, 0.09-0.11%, taken individually, of a tertiary element selected from the group consisting of Fe, Cr, Mo, V, Cu, Ni and W, the total of tertiary elements not exceeding 0.25%, up to 50 ppm of carbon, and the balance Zr.
Japanese Patent Publication 62-33734 (Takase et al) discloses a zirconium based alloy comprising, by weight, 1-2% Sn, 0.1-0.3 Fe, 0.05-0.2% Cr, 0.05-5% Mo, 0.05-5% Nb, 0.05-5% V and the balance Zr, apparently for use in boiling water reactors wherein the boiling water may be at a temperature of 500.degree. C. or higher. The corrosion resistance requirements differ for components used in boiling water reactors relative to components used in pressurized water reactors.
The present inventors conducted intensive studies in order to develop a zirconium alloy that would exhibit further improved corrosion resistance, strength and stress relaxation property (creep characteristics) when used as the constituent material of a pressurized nuclear reactor fuel assembly. As a result, the present inventors found that the inclusion of Nb and Ta led to a further improvement in corrosion resistance and that the incorporation of V and Mo resulted in an improvement in strength and stress relaxation property (creep characteristics). The resulting zirconium alloy was adapted for prolonged use as a nuclear reactor fuel assembly material.